Need some battery sizing advice


New Member
Hi All,

So first some info. I am in the UK, midlands / South West. The peak output from my PV in the summer is about 500 kw/h/month and drops to about 175 kw/h/month in the winter) The solar array is just under 5 kw peak, which feeds a hybrid grid-tied inverter (Solus RHI 5Kw 5G) which supports separate AC-in and AC-backup, 2 strings of PV max 8 kw @600V max, and various standard 48v battery banks (like LG and Pylon) and also programmable but dumb lead-acids. From the manual it can charge and discharge at 100A.

More info: my background consumption is about 1 kw/h constant day/night (several computers, networking, security system, fridge etc for which I will put in a new circuit to feed from the backup AC). My smart meter during sunshine usually indicates between 3-3.5kw being wasted (going to the grid for about 2p kw/h compensation :rolleyes:). My consumption usually goes up for a few hours in the evening as we like to watch TV/movies on a projector and that's quite hungry (another 4 or 500 W).

My main goal is to reuse the excess energy my array generates during the day to offset pulling from the grid at night. My secondary goal is to provide power to fridge/freezer, security, and networking infrastructure in the event of a blackout (which is quite rare in the UK these days - thankfully, and when it does it's usually resolved in a few hours at most).

So I need help with sizing a LifePO4 pack. As I see it, if capacity is too small you never get the full benefit from your solar array, and if too large you just waste money on batteries because you can never fully charge them... that's my simplistic impression anyway. The easiest thing for me to do is to order 16x 280ah or similar from China to make a 48v, ~13.5 kw/h pack and be done, or do I need to buy 32 cells (+ a couple of spares) for ~27 kw/h? Or something else?

I am wondering about buying Winston's (from EU) which are mega-expensive for comparable capacities but probably a good deal if quality/longevity is considered.

Oh I also need a (or maybe two) BMS, with CANbus. Currently the Seplos is at the top of my list followed by JK/Heltec. Would consider any advice here too.

Thanks a lot and sorry for the long post!


I See Electromagnetic Fields!
You would cycle the battery between the knees of the curve, up to 80% or 90% of gross capacity.
Sounds like one pack 48V 280Ah would be enough for 6 or 8 hours of average 1kW to 1.5kW, if you powered everything.
A full-size refrigerator draws around 1.5 kWh/day, so could run a couple of those and some other loads all day, and recharge battery from PV (at least with summer sun.) Winter, 5 or 6 kWh per day could still do it without too many extra loads.

Utility pays you 2p kw/h for surplus production ... what do they charge for consumption?

The cost of batteries, considering number of cycles until they wear out, may be high enough that you never save any money. Cheaper to just buy the power.
The cost I've estimated is $0.50/kWh for name-brand commercial lithium batteries, down to $0.05/kWh for DIY batteries built from 280 Ah LiFePO4 cells (that cost about $100 each).
And only that low if the battery lasts as long as claimed cycle life. In one test, 95% of commercial brands failed well before that.
So depending on your utility rates, it is possible DIY batteries will save you money, or maybe not.

With a small battery bank to supply motor-starting surge, or keep system running when a cloud passes over, you can run loads during the day mostly PV-direct. Small battery can also power datacom equipment at night.
Refrigeration could be shut off at night, to coast on the ice built up.

If not cycling batteries daily for load-shift function, rather only during occasional power failures, it is float life rather than cycle life which counts.
Some lithium batteries have design life of 20 or 30 years. Typically lead-acid is shorter, but some last 10 to 15 years or more if not cycled much, and price-point might make them competitive.

I use AGM for a grid-backup system. It should have 10 year or 700 cycles life, but probably won't even see 100 cycles. That makes cost per kWh high, but keeps the power on during grid failures and lets the (large) PV system power A/C and everything else. Inverter has signaling relays, could enable discretionary loads when battery above 80% SoC, when lower save power for more critical loads.


New Member
Utility pays you 2p kw/h for surplus production ... what do they charge for consumption?
From 1st Oct they are charging 20.5p kw/h.

To be fair, I think I get more than 2p per kw/h generated as they pay me to produce (a few pence) then they assume that I've fed half of that back to the grid (they can't tell how much apparently so they just make an assumption) and so pay me another 50% of the first payment on top. TBH I am not a fan of govt. hand outs, but I would like to be paid from the distributor a fair price for my surplus.

Thanks for the advice and good points.